Old tensions are resurfacing on the border between Syria and Turkey, across which roughly 12,000 Syrian refugees have fled to escape a deadly crackdown on anti-government protesters by Syrian President Bashar al-Assad.

Syrian troops have been massing near the border to keep more Syrian citizens from leaving, and Turkey, which is reportedly reinforcing its own troops in the area, is worried its neighbor could be preparing for cross-border strikes.

U.S. Secretary of State Hillary Clinton has called the development "very worrisome." Are Syria and Turkey headed for a violent clash?